Turbocharger compressors are characterized by a range of performance levels over a range of operating conditions. Typically this is graphically depicted on a compressor map, which plots the compressor pressure ratio against the corrected airflow levels for a range of design operating conditions. The compressor map defines a surge line and a choke line, which correspond to the varying extreme operating conditions at which the compressor will experience surge, i.e., at which significant intermittent backflow of air through the compressor will occur, and choke. Typically, compressor designs providing for a wider range of operating conditions prior to experiencing surge and choke are considered preferable.
A factor that can vary airflow levels for a single-sided compressor is the pressure of the inlet air at the compressor inducer. Other factors that can vary airflow levels are the geometry of the compressor wheel and the geometry of the diffuser.
With reference to FIG. 1, a single-sided compressor wheel 11 has two primary components, a hub 13 and a set of blades 15, each blade having a leading edge 17 that defines a compressor inducer at the upstream end of the passage through which the blades rotate, a trailing edge 19 that defines a compressor exducer at the downstream end of the passage through which the blades rotate, a hub edge 21 and a shroud edge 23. The each blade's shroud edge generally conforms to a housing shroud 25 with a small clearance.
Single-sided compressor wheel geometry can be significantly characterized by two parameters, the Trim, and the annulus area, which may be referred to as EI. Between two different single-sided compressor wheels, differences between these parameters (the Trim and/or the EI) will generally lead to single-sided compressors configured for different airflow levels (i.e., greater or lesser levels of airflow) for a given air pressure at the compressor inducer. In other words, the variations change the compressor maps. For example, it is known that larger trim numbers lead to greater flow levels.
The structural Trim of a single-sided compressor wheel is defined as follows:
  Trim  =                    D                  1          ,          S                2                    D        2        2              ×    100  As is seen in the figure, D1,S is the diameter of the shroud edge 23 of the (path of the) blades 15 at the inducer (i.e., where the shroud edge of the blades meets the leading edge 17), and D2 is the diameter of the wheel at the root end of the exducer (i.e., where the hub edge meets the trailing edge 19).
In an alternative aerodynamic approach, the aerodynamic TrimA is defined as follows: follows:
            Trim      A        =                            D                      1            ,            S                    2                          D                      2            ,            RMS                    2                    ×      100        where            D              2        ,        RMS              =                            1          2                ×                  (                                    D              2              2                        +                          D                              2                ,                tip                            2                                )                    and D2,tip is the diameter of the shroud edge 23 of the (path of the) blades 15 at the exducer (i.e., where the shroud edge of the blade meets the trailing edge 19). It should be noted that the structural trim and the aerodynamic trim are identical when D2,tip equals D2 (e.g., the trailing edge is parallel to the axis of rotation). Throughout this specification, the term Trim will refer the former of these definitions (the structural trim) unless the aerodynamic TrimA is expressly recited.
The annulus area of a single-sided compressor wheel is defined as follows:
  EI  =                              wheel          ⁢                                          ⁢          outlet          ⁢                                          ⁢          annulus          ⁢                                          ⁢          area                ,        E                              wheel          ⁢                                          ⁢          inlet          ⁢                                          ⁢          annulus          ⁢                                          ⁢          area                ,        I              =                  π        ⁢                                  ⁢                  D          2                ⁢                  B          2                                      π          ⁡                      (                                          D                                  1                  ,                  S                                2                            -                              D                                  1                  ,                  H                                2                                      )                          4            As is seen in the figure, D1,H is the diameter of the hub edge 21 of the (path of the) blades 15 at the inducer (i.e., where the hub edge meets the leading edge 17), and B2 is the axial width of the blades at the exducer.
Two housing walls, 31 & 33, define a single-sided compressor wheel diffuser 41, which is a passageway downstream of the compressor exducer. More particularly, the diffuser of a single-sided compressor is the radial passage extending from the compressor wheel exducer to a compressor volute 43, which is a spiral shaped air passage. The diffuser can be significantly characterized by the parameter DE, the vaneless diffuser annulus area ratio. For two identical single-sided compressor wheels having a given air pressure at their compressor inducers, variation of this parameter (DE) will generally cause the single-sided compressors to be configured for different airflow levels (i.e., greater or lesser levels of airflow), changing the compressor map.
The vaneless diffuser annulus area ratio of a diffuser for a single-sided compressor wheel is defined as follows:
  DE  =                              diffuser          ⁢                                          ⁢          outlet          ⁢                                          ⁢          annulus          ⁢                                          ⁢          area                ,        D                              wheel          ⁢                                          ⁢          outlet          ⁢                                          ⁢          annulus          ⁢                                          ⁢          area                ,        E              =                            D          3                ⁢                  B          3                                      D          2                ⁡                  (                                    B              2                        +            e                    )                    As is seen in the figure, D3 is the diameter of a downstream end 45 (outlet) of the diffuser 41 (i.e., where the airstream in the diffuser passageway enters the volute 43), B3 is the final (e.g., downstream end) axial width of the diffuser, and e is the axial distance between the shroud edges 23 of the blades 15 and the shroud 25 at the exducer (where the shroud edge meets the trailing edge 19, i.e., (B2+e) is the axial width of the passageway through which air flows at the exducer).
For various reasons, it is sometimes preferable to use a two-sided compressor wheel. For example, these wheels might have lower rotational inertia than a single-side wheel with a similar level of performance to the combined sides of the two-sided wheel. Alternatively, it might be preferable to have a lower level of axial load generated by the compressor wheel, as may be the case for two-sided compressor wheels. It is known to have a two-sided compressor having symmetric compressor wheel blades and a symmetric diffuser, each being symmetric across a plane of symmetry normal to a wheel axis of rotation (i.e., the middle plane of the hub backplate).
There exists a need for turbochargers having performance- and cost-efficient two-sided compressors. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.